Barn owls offer several advantages for the study of sound localization, including behavioral, neurophysiological and anatomical information about the localization cues they use. For the owls, the interaural time difference (ITD) encodes the horizontal angle and the interaural level difference (ILD) encodes the vertical angle of sound direction. The main goal of the present proposal is to fill the gaps in our knowledge about the processing of ITD and ILD by using new ideas and methods. The owl's brain processes ITD and ILD in separate pathways. All these processes are organized to produce the sensitivity for combinations of ITD and ILD in space-specific neurons. The nucleus laminaris is the first site of ITD detection. It detects ITDs in separate frequency bands in which each neuron is tuned to a narrow range of frequencies. Our preliminary results show that we can predict the frequency tuning curve of a laminaris neuron from its responses to ITDs, because the neuron simply adds ITD responses for different frequencies. This finding indicates that laminaris neurons carry out cross-correlation of inputs from the two sides. We plan to extend this analysis to high-order ITD sensitive neurons to determine the stage where the additive process is replaced by non-linear integration of inputs from different frequency bands. We already know that the ITD and ILD combination sensitivity of neurons of the external nucleus of the inferior colliculus (ICx) involves a multiplication of postsynaptic potentials from the ITD and ILD pathways. We plan to determine the biophysical bases for multiplication. ICx projects mainly to the optic tectum but not to the thalamus so far as we know. However, both the thalamus and forebrain contain space-specific neurons. The question is how two independent brain areas come to contain neurons with the same response properties. Our preliminary results suggest that the thalamic-forebrain space-specific neurons are different from the ICx counterparts. We propose to pursue this line of research further in this proposal.